Automatic frequency control circuits



June 28, 1949.

Filed June 4, 1943 G. GUANELLA AUTOMATIC FREQUENCY CONTROL CIRCUITS 2Sheets-Sheet 1 gian June 28 1949. G. GUANELLA 2,474,354

AUTOMATIC FREQUENCY CONTROL cIRcUITS Filed June 4, 1943 2 Sheets-Sheet2l f9.4. Z 3mm? I @w awww.

l'atelltd June Z5, 1343 AUTOMATIC FREQUENCY CONTROL CIRCUITS GustavGuanella, Zurich, Switzerland, assignor to Patelhold Patentverwertungs-& Electro- Holding A.-G., Glarus, Switzerland Application June 4, 1943,Serial No. 489,679 In Switzerland May 19, 1942 Section 1, Public Law690, August 8, 1946 Patent expires May 19, 1962 (Cl. Z50-40) 13 Claims.

With radio transmitting and receiving apparatus, particularly for veryhigh frequencies, certain diflculties are encountered as regards thetuning and the avoidance of uncontrollable frequency deviations. Theemployment of eX- tremely frequency-constant control elements, such asfor instance oscillation crystals with a very small natural damping,certainly enables the frequency to be stabilized. Besides the frequencymultiplying arrangements especially necessary with short waves such asolution, however, involves Very complicated measures if the frequencyof the radio apparatus has to be varied over a more or less wide range.

The present invention concerns methods of and circuit arrangements forproducing a control voltage by which it is always possible to maintainthe tuning error of transmitting and receiving equipment at a minimum.The new methods and circuits have the advantage that they do notdetrimentally aect the quality of the transmis, sion, either of anunmodulated signal or of a signal carrying an amplitude or a frequencymodulation.

The control voltage for automatic tuning is obtained by periodicallyvarying the tuning of an oscillator or of an oscillatory circuit workingout of the oscillator, rectifying the output voltage of the oscillatorycircuit to develop an alternating voltage that varies in amplitude withthe instantaneous detuning of the circuit, and phase-rectifying thealternating voltage of varying amplitude to develop a pulsating voltagehaving a mean value of an algebraic sign that corresponds to the senseof the prevailing average detuning. The frequency of the applieddetuning voltage should be outside of the range of transmission of theradio apparatus in which the oscillatory circuit is incorporated, andmay be subsonic or supersonic. When the oscillatory circuit isaccurately tuned to the desired signal frequency, the developed controlvoltage is of course zero.

Objects of the invention are to provide methods of and circuitarrangements for automatic tuning that are characterized by a highfidelity transmission over a wide tuning range. Objects are to provideautomatic tuning methods and circuit arrangements that are applicableboth to amplitude and to frequency modulated transmission and reception.More specifically, objects are to provide methods of and circuitarrangements for cyclically varying the detuning of an oscillatorycircuit by superposing an alternating current upon a voltage-variantreactance of the circuit, and developing from the resultantamplitude-varying output a pulsating current that varies in magnitudeand in algebraic sign with the average detuning of the circuit.

These and other objects and advantages of the invention will be apparentfrom the following specification when taken with the accompanyingdrawings in which:

Fig. 1a is a resonance curve of an oscillatory circuit of the apparatusto be controlled;

Fig. 1b is a curve sheet showing on a time basis the cyclic detuningthat arises when an oscillatory current or voltage input to the circuitis continuously varied over a small frequency range;

Fig. 1c is a curve sheet showing on a time basis the amplitude variationof the voltage output which results from the cyclic detuning;

Fig. 1d is a curve sheet showing on a time basis the magnitude andalgebraic sign of the pulsating control voltage developed from theamplitudevarying output voltage;

Figs. 2 and 3 are schematic diagrams of circuit arrangements fordeveloping control voltages that vary in amplitude and in sign with theextent and the sense of the average detuning of an oscillatory circuit;and

Fig. 4 is a circuit diagram of a radio transmitter that includeselectrical devices for developing the desired control voltages forautomatic tuning.

The resonance curve of a frequency-stabilized control circuit such ascontemplated by the invention is shown in Fig. 1a, the abscissae F ofthe curve being the magnitude of the detuning (fz-f1) between thefrequency f1 of the transmitted oscillations and the resonant frequencyf2 of the control circuit. The ordinates of the curve indicate therelative amplitudes of the current or voltage output of the circuit forsignals of different frequencies below and above the resonant frequency,f2 of the control circuit. As illustrated, the frequency f2 of thetransmitted oscillations differs from the resonant frequency f1 of thecircuit by a value F0 and, in accordance with the invention, the tuningerror is varied between values F1 and F2 by imposing a periodicvariation upon the frequency f1 of the transmitted oscillations, seeFig. 1b. A voltage output from the control circuit may be rectified todevelop an alternating current u of the frequency of the applieddetuning, see Fig. 1c, which .varies in amplitude between values u1 andu2 with respect to a value un corresponding to the average detuning F0.By rectifying this alternating voltage u. a pulsating control voltage pis obtained which has a mean value q dependent upon the average detuningFo, see Fig. 1d, and which has an algebraic sign corresponding to thesense of the detuning when the rectification is effected by aphase-sensitive rectifier. This average value q of the control voltagebecomes zero as soon as the average detuning disappears.

An arrangement for performing the method according to the invention isillustrated schematically in Fig. 2 as including as oscillator G whichsupplies a radio frequency voltage to the oscillatory circuit S which istuned to resonance at a frequency f2, the basic frequency f1 of theoscillator being varied at a low frequency fx by an alternating currentvoltage v developed by a source N. The specific details of the circuitelement or elements by which the frequency of the oscillator is variedbetween f1-fx and fz-l-fx by the applied voltage v are not an importantfeature of the invention. Any of the known arrangements, for

j example a saturating core inductance, a conf denser having anelectrode vibrated by an elec- ;1 tromagnet on which the voltage "u isimpressed,

@tor a variable reactance tube may be employed.

The frequency f1 ifx of the radio frequency signal transmitted by thecontrol circuit S is not affected by the mistuning of that circuit tofrequency f2 but the amplitude of the signal at the output side of thecircuit S varies at the applied mistuning frequency fx. A rectier D isconnected to the oscillatory circuit S and develops a low frequencyalternating voltage u which varies in amplitude with the instantaneousdetuning as shown in Fig. 1c. This alternating voltage u is of the samefrequency fx as the applied detuning voltage v, and is imposed upon aphase-sensitive rectifier M, which is shown schematically as a ringmodulator upon which the alternating voltage o is also impressed. Thevoltage output p from the ring modulator M is a pulsating direct currentvoltage which varies in magnitude and also as to algebraic sign, withthe periodic detuning of the generator G. The pulsating voltage p istransmitted through a low pass filter B to develop an average controlvoltage q which is impressed upon the control oscillatory circuit S, asindicated by the dotted line connection, to adjust the resonantfrequency thereof towards the mean frequency ,f1 of the signal voltageoutput from the generator G. The value of the control voltage q may beindicated on a measuring instrument J.

In an alternative arrangement, as shown schematically in Fig. 3, theperiodic detuning by the voltage 1J is imposed upon the tunedoscillatory circuit S, and the control voltage q, which is developed bythe same circuit elements and process steps as in the Fig. 2 circuit, isapplied to generator G to reduce the divergence between the frequency f1of the generator output and the resonant frequency f2 of the tunedcircuit S.

The periodic influence of the tuning of the generator G or theoscillation circuit S by the control voltage v is for instance achievedwith controllable reactances such as variable reactance tubes,inductances with variable saturation of the iron core, and the like.Purely mechanical tuning controls for the oscillator or the oscillationcontrol circuit are also possible if, for instance, small additionalcapacities are provided the magnitude of which is varied byelectomagnetic devices responsive to the regulating voltage. The samealso applied to the tuning influences which the control voltage q has toexert.

Fig. 4 shows the application of the invention to a frequency oramplitude modulated transmitter.

The controlled oscillator G1 of the transmitter can for the purpose offrequency modulation (FM) be influenced by the low frequency signal Wf.The tuning of the frequency-control circuit S1 is varied periodically insynchronism with the low frequency voltage v by means of a wobblingcircuit W1, so that the output amplitude of this circuit is variedperiodically by known frequencyvariant reactances such as justdescribed. The voltage output of the control circuit S1 is rectied. aspreviously described, in a rectifier D1 to develop a voltage u of thesame frequency as the detuning voltage v. With a frequency modulation ofthe generator G1, the low frequency voltage u contains a considerablenumber of undesirable components whose frequency differs from theconstant frequency of the regulating voltage o. A band-pass filter P1 isprovided for the suppression of these disturbing components, Afteramplification in amplifier V1 the voltage u which depends on thedetuning is passed to the phasesensitive rectifier circuit M1 at theoutput of which the voltage p1 occurs in the manner described. Thecontrol voltage q1 obtained by smoothing p1 in the low-pass filter B1acts on a tuning element of the oscillator G1 in such a manner that itsaverage detuning compared with the control circuit S1 always remains assmall as possible.

By this means a frequency modulated high frequency voltage e1 can beobtained whose average frequency always corresponds to the meanresonant-frequency f2 of the circuit S1. The tuning variations of theoscillator G1 due to voltage and temperature changes, which aredifficult to avoid, are thus always immediately corrected. The controlcircuit S1 can be constructed with a very small damping and can alsoeasily be maintained at a constant temperature so that the frequency ofthe generator G1 remains to a great extent independent of externalinfluences. The tuning of the control circuit S1 and if necessary at thesame time also the rough tuning of the generator G1 are varied for thepurpose of selecting a desired frequency.

The same method is also used for a tuning control oi the couplingcircuit S2. For this purpose the tuning of this circuit is variedperiodically by the wobbling device W2. With the aid of the intermediatecircuits D2, P2, V2, Mz, Bz whose construction and method of operationcorrespond with that of the circuits already mentioned, a controlvoltage q2 is obtained which maintains the detuning of the couplingcircuit S2 continuously at a minimum. The same method can also beapplied to the further coupling circuit S3. Between the controlledcircuits are the individual amplifier stages A1, A2 whose amplificationcan be controlled by a low frequency signal Wa for the purpose ofamplitude modulation (AM), as is indicated at A2.

Due to the periodic tuning variations of the control and couplingcircuit S2 small periodic variations occur in the amplitude of the highfrequency output voltage e2. These variations affect the automatictuning control of the oscillat1on circuit S3 if its wobbling occurs inthe same or opposite phase as the tuning variation of S3. This influencecan, however, be avoided by shifting the phase of the low frequencyvoltage v by in the phase-shifting circuit Q. Another means of avoidingthis disturbance is to use different frequencies for the voltages owhich serve to alter periodically the tuning of the coupling circuits S2and S3.

The same method can of course also be applied to numerous otherarrangements, particularly with radio receivers. Where superheterodynereceivers are concerned it is advisable to influence the auxiliary orlocal oscillator by means of both the control voltage q as well as theperiodically varying regulating voltage v, so that the detuning of theintermediate frequency signal compared with the tuned intermediatefrequency circuits is maintained at a minimum.

I claim:

1. In radio receiving or transmitting apparatus, a pair of seriallyarranged oscillatory circuits comprising a tunable carrier frequencyoscillator working into a tunable control circuit normally resonant atthe desired frequency of carrier Wave generation, said oscillatorincluding a voltagevariant reactance to control the resonant frequencythereof, means for cyclically varying the resonant frequency of saidcontrol circuit about its normal value of the desired frequency ofcarrier wave generation, whereby the output voltage of said circuitsvaries periodically in amplitude with the cyclic detuning of saidcontrol circuit, means for rectifying said amplitude-variant voltage todevelop an alternating voltage of the frequency of the periodicdetuning, means for phaserectifying such alternating voltage to developa pulsating voltage having a mean value dependent in magnitude and signupon the degree and sense of the average detuning of said circuits withrespect to each other, and means for impressing said pulsating voltageupon the voltage-variant reactance in said oscillator circuit to reducethe average detuning.

2. In radio receiving and transmitting apparatus, the invention asrecited in claim 1, wherein said means for cyclically varying the tuningof said control circuit includes a variable reactance in said circuit,electro-mechanical means for controlling the effective value of saidvariable reactance, and a source of cyclically varying voltageenergizing said electromechanical means.

3. In radio receiving and transmitting apparatus, the invention asrecited in claim l, wherein said means for phase-rectifying suchalternating voltage comprises a ring modulator.

4. In radio receiving or transmitting apparatus, a source of oscillatorycurrent, a plurality of cascaded transmission stages working out of saidsource and each including an oscillatory circuit having avoltage-variant reactance, means for imposing different periodicdetunings upon said oscillatory circuits to develop voltages fluctuatingin amplitude with said detunings, and means including phased rectiersfor developing from said fluctuating-amplitude voltages a plurality ofcontrol voltages for application to the voltage-variant reactances ofthe associated oscillatory circuits to reduce the average detuningthereof With respect to the frequency of said oscillatory currentsource.

5. In radio receiving or transmitting apparatus, the invention asrecited in claim 4, wherein said means for detuning said oscillatorycircuits impose detunings thereon of different frequencies.

6. In radio receiving or transmitting apparatus, the invention asrecited in claim 4, wherein said means for detuning said oscillatorycircuits impose detunings thereon of the same frequency but of differentphase.

'7. In radio receiving or transmitting apparatus, the invention asrecited in claim 4, wherein said means for detuning said oscillatorycircuits impose upon two of said oscillatory circuits detun- 6 ings ofthe same frequency and phase-shifted by 8. In the operation of radiotransmitting and receiver apparatus including a tunable carrierfrequency oscillatorworking into a tunable oscillatory circuit, saidoscillator including a voltagevariant reactance for controlling theresonant frequency thereof, the process of reducing any tuning deviationbetween the resonant frequency of said circuit and the frequency of theoscillator which comprises the steps of periodically and progressivelyvarying a frequency-controlling voltage applied to the voltage-variantreactance of said oscillatory circuit to effect a cyclic tuningdeviation at a frequency outside the signal transmission range of theapparatus, thereby to vary the amplitude of the voltage output of saidcircuit at the frequency of tuning deviation, rectifying suchamplitude-varying voltage to develop an alternating current voltage ofthe frequency of tuning deviation, phase-rectifying the alternatingcurrent voltage to develop a direct current voltage which varies inalgebraic sign with the sense of the average detuning, and impressingsaid direct current voltage upon the voltage-variant reactance of Saidoscillator.

9. In the operation of radio transmitting and receiving apparatus of thetype including a pair of cascaded tunable circuits, each of saidcircuits including a voltage-variant reactance to control the tuningthereof, the process of reducing deviations between the resonantfrequencies of said circuits with respect to the frequency of a carrierwave transmitted by said circuits, said process comprising the steps ofperiodically and progressively varying the tuning of said circuits at afrequency outside the audio frequency range, thereby to vary theamplitudes of the output voltages of said cascaded circuits at thefrequency of the imposed detuning, rectifying such amplitude-varyingvoltages to develop alternating voltages of the frequency of the applieddetuning, phase-rectifying such alternating voltages to develop directcurrent voltages which vary in algebraic sign with the sense of theaverage detunings of the respective circuits, and imposing such directcurrent voltages upon said voltagevariant reactances of the respectivecircuits from which they were derived to reduce the average detuningthereof.

10. In the operation of radio transmitting and receiver apparatus of thetype including a plurality of tunable circuits in series in a line ofsignal transmission, each of said circuits including voltage-'variantreactances for controlling the resonant frequency thereof; the processof controlling the tuning of said circuits which comprises periodicallyvarying the resonant frequencies of said tunable circuits at differentfrequencies which are each outside the range of audio frequencies,rectifying the output voltages of said circuits to develop alternatingcurrent voltages of the said different frequencies, phaserectifying saidalternating current voltages to develop direct current voltages whichvary in algebraic sign with the sense of the average detuning of theseveral circuits, and impressing said direct current voltages upon thevoltagevariant reactances of the individual circuits from which therespective direct current voltages were derived.

11. In the operation of radio transmitting and receiver apparatus of thetype including a plurality of tunable circuits in series in a line ofsignal transmission, each of said circuits including voltage-variantreactances for controlling the resonant frequency thereof; the processof controlling the tuning of said circuits which comprises periodicallydetuning said circuits at the same frequency but out of phase,rectifying the output voltages of said circuits to develop alternatingcurrent voltages, phase-rectifying said alternating current voltages todevelop direct current voltages which vary in algebraic sign With thesense of the average detuning of the several circuits, and impressingsaid direct current voltages upon the voltage-variant reactances of theindividual circuits from which the respective direct current voltageswere derived.

12. In the operation of radio transmitting apparatus of the typeincluding a., tunable carrier frequency oscillator Working into anamplifier stage and a control circuit having a resonant frequencycorresponding to the desired frequency of carrier Wave transmission,said oscillator including a voltage-variantreactance for controlling theresonant frequency thereof, means for impressing a modulation upon thecarrier frequency output of said oscillator, and a tunable circuitincluding a voltage-variant reactance coupling said oscillator and saidamplifier stage; the process of maintaining a substantially constantfrequency transmission which comprises periodically varying the resonantfrequency of said control circuit and said coupling circuit at afrequency outside the modulation range to develop alternating voltageoutputs therefrom which vary in magnitude at the frequency of thedetuning, phase-rectifying such alternating current voltages to developpulsating direct current voltages which vary in magnitude and inalgebraic sign with the extent of and the sense of, respectively, thedetuning of the oscillator and the coupling circuit, impressing thedirect current voltage derived from said control circuit upon thevoltage-variant reactance of said oscillator, and impressing the otherdirect current voltage upon the voltage-variant reactance of saidcoupling circuit, whereby said oscillator and coupling circuit may beretuned to the desired transmission frequency.

13. In the operation of radio transmitting and receiving apparatusincluding a plurality of cascaded amplifier stages upon which a carrierWave of substantially fixed frequency is impressed, and couplingcircuits including voltage-variant reactances Working into saidamplifier stages; the process of reducing detuning of said couplingcircuits which comprises periodically detuning said coupling circuits,thereby to develop alternating output voltages therefrom of thefrequency of the detuning, phase-rectifying such alternating outputvoltages to develop direct current voltages which vary in magnitude andsense with the average detuning of the coupling circuits, and impressingsuch direct current voltages upon the voltage-variant reactances of theindividual coupling circuits to reduce the average detuning thereof.

GUSTAV GUANELLA.

REFERENCES CITED The following referenlces are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,819,904 Love Aug. 18, 19311,907,965 Hansell May 9, 1933 2,231,996 Guanella et al Feb. 18, 19412,245,685 Koch June 17, 1941 2,261,800 Freeman Nov. 4, 1941 2,287,925White June 30, 1942 2,296,100 Foster et al Sept. 15, 1942 2,354,510 EarpJuly 25, 1944 OTHER REFERENCES 363,862 Dolle et al Pub. May 25, 1943

